This invention relates to a method for removing impurities, such as magnesium, from wet process phosphoric acid.
Phosphoric acid has been prepared by the wet process method for many years. The wet process involves the reaction of phosphatic solid materials, hereinafter termed phosphate rock, with sulfuric acid, usually in a slurry of phosphate rock and calcium sulfate in phosphoric acid. The sulfuric acid reacts exothermically with the phosphate rock to produce a slurry of phosphoric acid and calcium sulfate. The phosphoric acid is separated from the calcium sulfate by filtering.
The names of three processes for producing phosphoric acid are based upon the by-product calcium sulfate produced, namely, the gypsum or dihydrate process, the hemihydrate process, and the anhydrite process. The type of by-product produced is dependent upon the temperature of the system and the P.sub.2 O.sub.5 concentration of the liquid. Other factors, such as fluorine concentration, alumina concentration, and sulfuric acid concentration, play a less important role. As used herein, "calcium sulfate" refers to all three types of calcium sulfate, i.e., gypsum, hemihydrate, and anyhydrite.
Gypsum, CaSO.sub.4 . H.sub.2 O, is the by-product formed when the wet process is run at a temperature of 90.degree. C. or less and a P.sub.2 O.sub.5 concentration of about 30% in the liquid portion of the slurry. Increasing the temperature to about 90.degree. to 120.degree. C. and the P.sub.2 O.sub.5 concentration to about 40% in the liquid phase yields hemihydrate, CaSO.sub.4 . 1/2H.sub.2 O. The phosphoric acid produced by the hemihydrate process, before any concentration, typically has a P.sub.2 O.sub.5 content greater than 38% by weight. CaSO.sub.4, anhydrite, is produced at temperatures of about 130.degree. C. at P.sub.2 O.sub.5 concentrations greater than 30%. This latter process is most difficult to run due to severe corrosion at the higher temperatures and the instability of the anyhydrite during processing.
A detailed description of a method for producing phosphoric acid by the dihydrate process can be found in U.S. patent application Ser. No. 909,899, filed by Fernando Ore, John David Ellis, and James Harold Moore, entitled "Phosphoric Acid Processed with High Circulation Rates", which is a continuation-in-part of U.S. patent application Ser. No. 703,138, filed July 7, 1976 now abandoned. Both of these applications are incorporated herein by this reference.
A process for preparing phosphoric acid by the hemihydrate process is described in U.S. patent application Ser. No. 865,556, filed Dec. 29, 1977, by Fernando N. Ore, John David Ellis, and James Harold Moore, entitled "Hemihydrate Type Phosphoric Acid System", which is a continuation-in-part of application Ser. Nos. 703,139 and 703,138, both of which were filed on July 7, 1976. All three of these applications are incorporated herein by this reference.
When phosphoric acid is made by any of these processes, impurities in the rock are dissolved and are present in the resulting phosphoric acid. The principal impurities which are difficult to remove are compounds and complexes containing calcium, aluminum, silicon, fluorine, sulfate, and magnesium. Removal of these impurities is important because the phosphoric acid is usually concentrated up to about 48 to 54 weight percent P.sub.2 O.sub.5 and, during storage, the impurities can precipitate to form a solid which is found objectionable by most customers.
Many attempts have been made to remove impurities from wet process phosphoric acid. Exemplary of the techniques used are those described in U.S. Pat. Nos. 3,124,419; 3,206,282; 3,273,713; 3,379,501; 3,442,609; 3,481,700; and 3,642,439; and British Patent Nos. 467,843 and 1,337,669. Methods described in these patents generally are complicated, difficult to use, and in many cases are ineffective for removing magnesium-containing impurities. In addition, none of the methods described in these patents involves a commercial use for the precipitate formed in removing the impurities from phosphoric acid.
Therefore, there is a need for a simple, inexpensive, and effective method for removing impurities, such as magnesium, from wet process phosphoric acid, and there is a need for a method which converts the precipitate formed into a useful product.